The approach that I would suggest uses Icenium Mist:
Export a stable version of your app's source and import it in a clean project (or if using github/bitbucket clone your project again);
Build the app for Android in Mist and collect the QR code that you will be provided with;
Get the QR code for iON (Run -> OnDevice -> iOS -> No Provisions);
Get the QR code for your project that will be used by iON (thePathAbove -> "I have installed iON already...").
Having the 3 QR codes you can send them with a brief description (Android: allow install from unknown sources and direct scan; iOS: scan iON QR code and install iON and scan the app QR code).
I recommend demoing with a separate project because, this way you will be sure that your testers will not get between stable versions of your code, since with both iON on iOS and regular Debug build in Android we provide sync (iOS: 3-finger hold, Android: pressing the settings menu button) and if you work on the project that they sync to, they will be getting your changes.
The best way is to have your project on git repo host (github, bitbucket, etc.) and to clone the project when it's stable to a second project.
Apart from the QR codes there is a second way to distribute demos:
Android: download the .apk produced by the build and directly send the binary;
iOS: collect the UDIDs on which the app will be tested, add them to a development provision of yours and build with this provision, distributing as above (download and send).
Again I would suggest to practically copy your project for stability of the demo.